Abstract
Amorphous calcium carbonate (ACC) is commonly found in many biological materials. As ACC readily crystallizes into calcite, stabilizers, such as anions, cations or macromolecules, often occur to avoid or delay unwanted crystallization. In biogenic ACC, magnesium is commonly present as one of the stabilizing agents. It is generally thought that the presence of mobile water in ACC is responsible for its limited stability and that the strong interaction of Mg2+ with water stabilizes the amorphous structure by retarding dehydration of ACC. To test this hypothesis, we studied the mobility of hydrous species in the model materials ACC, amorphous magnesium carbonate (AMC) and amorphous calcium/magnesium carbonate (ACMC), using quasi elastic neutron scattering (QENS) which is highly sensitive to the dynamics of H atoms. We discovered that hydrous species in the considered amorphous materials consist of water and hydroxide ions, as magnesium ions are incorporated in a ratio of 1 to about 0.6 with OH-. Surprisingly, we found that there is no evidence of translational diffusion of water and hydroxides when calcium is present in the samples, showing that hydrous species are highly static. However, we did observe diffusion of water in AMC with similar dynamics to that found for water in clays. Our results suggest that Mg2+-water interactions alone are not the only reason for the high stability of AMC and ACMC. The stabilizing effect of Mg ions, in addition to Mg-water binding, is likely to be caused by binding to hydroxide in amorphous calcium carbonates. In fact, the incorporation of hydroxides into the amorphous phase results in a mineral composition that is incompatible with any of the known Ca/Mg-carbonate crystal phases, requiring large scale phase separation to reach the composition of even the basic magnesium carbonate minerals artinite and hydromagnesite.
Highlights
Amorphous calcium carbonate (ACC) to calcium free amorphous magnesium carbonate (AMC)using quasi elastic neutron scattering (QENS)
To assess the composition of the prepared materials, the samples were measured with inductively coupled plasma-optical emission spectroscopy (ICP-OES), thermogravimetric analysis (TGA) (Fig. S1, Electronic supplementary information (ESI)†) and Fourier transform infrared spectroscopy (FTIR) (Fig. 1A)
It was previously shown that hydroxides are only incorporated in ACC in trace amounts unless NaOH is added during the synthesis,[16] which was not the case of the samples studied here
Summary
ACC to calcium free amorphous magnesium carbonate (AMC). The QENS instrument IRIS23 was chosen given its wide dynamic range from a few ps to tens of ps, the range used to study translation diffusion in bulk and confined water in many mesoporous materials.[24,25,26,27] This range covers the time scales for confined rotational motion of hydrous species in both clays and zeolites.[26,28] Compositional analysis of the samples revealed that, unlike calcium ions, magnesium ions coprecipitate with hydroxide ions generated by the sodium carbonate salt upon 19682 | Phys.
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